Measuring blood pressure is very important in antihypertensive therapy. According to the guidelines for antihypertensive therapies of WHO/ISH, the degrees of hypertension are classified in accordance with blood pressure values measured every 5 mmHg, and therapeutic methods suitable for the individual degrees are recommended. Therefore, whether appropriate therapies can be performed depends on the measured blood pressure values. Also, as the aging population advances, demands for high accuracy and high reliability of blood pressure measurement are on the rise in order to prevent circulatory organ diseases and metabolic syndrome of which hypertension has a large effect on.
Conventionally, as measurement methods of a non-invasive sphygmomanometer for measuring the blood pressure by winding a cuff around a blood pressure measurement portion, and gradually changing the cuff pressure from a pressure higher than the systolic blood pressure (also called a maximum blood pressure) to a pressure lower than the diastolic blood pressure (also called a minimum blood pressure), the microphone method that measures the blood pressure by detecting the Korotkoff sounds as in the auscultatory method and the oscillometric method that measures the blood pressure by detecting the change in pulse wave superposed on the internal pressure of an internal air bladder of a cuff are used.
In the oscillometric method, for example, when the cuff pressure is gradually changed from a pressure (for example, 180 mmHg) equal to or higher than the systolic blood pressure value to a pressure (for example, 60 mmHg) equal to or lower than the diastolic blood pressure value, the amplitude of a detected pulse wave initially shows an almost constant value but gradually increases as the cuff pressure approaches the systolic blood pressure value. When the cuff pressure becomes equal to or lower than the systolic blood pressure and approaches the diastolic blood pressure, the amplitude of the pulse wave reaches its maximum and then gradually decreases. Furthermore, when the cuff pressure becomes equal to or lower than the diastolic blood pressure value, the amplitude of the pulse wave gradually changes to approach a certain predetermined value. In the oscillometric method, therefore, a time-series pulse wave amplitude change profile corresponding to the change in cuff pressure is normalized as is indicated by the ratio (%) of each pulse wave amplitude based on a maximum pulse wave amplitude of the magnitude of a detected pulse wave. At the same time, the ratio of a pulse wave corresponding to the systolic blood pressure value and diastolic blood pressure value measured by the auscultatory method (K method) is obtained from the average value of many actual data. This value is 50% for the systolic blood pressure, and 60% to 80% for the diastolic blood pressure.
Unfortunately, the relationship between the blood pressure value obtained by the auscultatory method and the ratio of the pulse wave amplitude described above is influenced by individual differences in blood pressure value, pulse strength, and shape of the invasive blood pressure waveform as the blood vessel internal pressure. This relationship is also influenced by the factors of measurement methods, for example, the variation in cuff-edge effect (a phenomenon in which the blood vessel pressing force in the end portions of a cuff is weaker than that in the central portion) in the upstream and downstream portions of a cuff, which is caused by the way the cuff is wound, and the change in compliance (the change in pulse wave detection sensitivity) caused by the way the cuff is wound. A cardiac output phenomenon in a cuff peripheral portion is influenced by individual differences in blood vessel elasticity and blood vessel volume in the forearm and hand as peripheral portions of a cuff attachment portion, the individual differences in degree of the peripheral circulation of the blood after the blood pressure is measured, and the rise in peripheral blood vessel internal pressure due to the degree of congestion of the blood vessel on the cuff peripheral side, which is affected by the shortness of the repetition time of blood pressure measurement.
Most of the influencing factors are problems caused by individual differences, and difficult to directly control. Controlling the way a cuff is wound has a large effect on the usability of blood pressure measurement. Therefore, as a method of reducing the influencing factors in the measurement of the systolic blood pressure, the S/N ratio is increased by raising the sensitivity of detection of a pulse wave output to the cuff peripheral side. For example, patent reference 1 has proposed a double-cuff method that improves the ability to detect a pulse wave output to the cuff peripheral side, which is the key point of systolic blood pressure measurement, by installing a pulse wave detection air bladder for selectively detecting the cardiac output on the cuff peripheral side, in a cuff central portion where the pressure of a compression air bladder is most reflected.
PLT1: Japanese Patent Laid-Open No. 2005-185295